website: Up supported clang version to 11

Change-Id: Ibeb5200a7919aa3fb38399305bf355805e603e69
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/48723
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>
1 file changed